Plastic encapsulated integrated circuits have been found to suffer damage due to stresses caused by thermal expansion and contraction in the molding compound surrounding the integrated circuit. Stressing, resulting from temperature cycling of the chip and other thermal coefficient of expansion mismatches of the materials, causes mechanical stresses in the plastic encapsulated integrated circuits. The highest of these stresses occur near the comers of the encapsulated die. The mechanical force exerted on the comers of the die results in several types of damage in the die, which are discussed below. The amount of damage found in the integrated circuit is a partly a function of the layout strategy of the area near the comer of the die.
As a result of the thermal expansions and contractions and resulting mechanical stresses in the molding compound, delamination, or movement of the molding compound with respect to the surface of the die occurs. When the thermally induced stresses are great enough to produce fractures in the molding compound, the area of damage to the die may be increased, and thin film cracking on the die is possible. As might be expected, such damage to the die, as a result of the stress, can cause the performance of the plastic encapsulated integrated circuit device to suffer, or cause the entire circuit to fail.
Previous attempts to solve the problem of molding compound movement have focused on comer structures on the die composed of metal and contact layers. In such structures, metal and contact layers are placed onto the comer of the die to act as a bonding area for the molding compound, as will be shown in conjunction with FIG. 1. However, metal and contact layers have a limited depth or topological variance, and thus, a lower ability to lock the molding compound to the die. Additionally, the comer structures do not reduce the threat of mold compound fractures.
Another problem facing plastic encapsulated integrated circuits is delamination. Delamination occurs when the molding compound separates from the surface of the die. If delamination occurs, and the separation is large enough, the topological variations of the comer structures can no longer restrain the movement of the molding compound with respect to the die surface. When movement of the molding compound with respect to the surface of the encapsulated die occurs, thin film cracking as well as damage beyond the area covered by the comer structure is possible.
Attempts to alleviate the problem of molding compound movement by utilizing comer structures, have an additional drawback. Comer structures placed onto the surface of the die reduce the available active die area.
A method or means is needed which reduces molding compound fracturing, prevents movement of the molding compound with respect to the die surface, and does not require using active die space to achieve these results.